1. Field of the Invention
The present invention relates to a sense amplifier, and more particularly to a sense amplifier with a larger sensing voltage range.
2. Description of the Related Art
Non-volatile memory is the major memory product can conserve data without requiring power over a long period of time. In Magnetoresistive Ram (MRAM) and Resistive Ram (RRAM) for example, the logic state of data stored therein is determined by the resistance thereof. Because the resistance of logic state 1 and 0 are different, a sense amplifier is applied to sense the current of the memory cell by applying a fixed voltage thereon, thus, the accuracy of determining the logic state of the data stored in memory depends on the performance of the sense amplifier. Furthermore, the reading speed of memory is relevant to the performance of the sense amplifier. The shorter reading time is desirable and the reading time is also relevant to the current passing through the memory cell.
FIG. 1 is a circuit diagram of a conventional sense amplifier. Control current source 11 is coupled to a high voltage source VDD and the drain and the gate of transistor N11. Resistor R is coupled to the source of transistor N11 and a ground. Transistor P11 has a source, a drain and a gate, wherein the source is coupled to the high voltage source VDD, the gate is coupled to the gate of transistor P12 and the drain is coupled to the drain of transistor N12. Transistor P12 has a source, a drain and a gate, wherein the source is coupled to the high voltage source VDD and the drain is coupled to the drain of transistor N13. The gate of transistor N13 is coupled to the gate of transistor N12. Memory Cell 12 is coupled to the source of transistor N12 and ground. Reference memory cell 13 is coupled to the source of transistor N13 and ground. Memory cell 12 has an equivalent resistance Rcell and when a predetermined voltage is applied to the memory cell 12, the current of the memory cell 12 is Icell. The reference memory cell 13 has two parallel resistors Rmax and Rmin, and when the predetermined voltage is applied to the reference memory cell 13, the current of Rmax is IH and the current of Rmin is IL. Control current 11 outputs a control current Ibias which utilizes the different W/L values of transistors P11 and P12 (In FIG. 1, the W/L value of the transistor P12 is twice the W/L value of the transistor P11) to control the current flowing in through the transistor P12 to be Iref and the current flowing in through the transistor P11 to be
      1    2    ⁢            I      ref        .  Resistors Rmax and Rmin respectively have resistance when the data stored in memory cell 12 stores is logic 1 or logic 0, and when the predetermined voltage is applied to the memory cell 12, the current passing through the memory cell 12 is respectively the current IH or IL In FIG. 1, the data stored in memory cell 12 is determined by applying the predetermined voltage to the memory cell 12 to make the memory cell current Icell be IH or IL, thus a differential voltage between node 14 and 15 is generated and a comparator 16 receives the differential voltage to determine what data is stored in the memory cell 12. In FIG. 1, the reference current Iref is the sum of IH and IL, thus a half divider circuit is required and the layout area of the sense amplifier is increased. Moreover, the difference between the memory cell current Icell and the reference current Iref is not easily determined, thus, the speed and accuracy of the sense amplifier suffers. In FIG. 1, the current sensing range is
            1      2        ⁢          (              IL        -                  I          H                    )        or            1      2        ⁢          (                        I          H                -                  I          L                    )      and if the difference between IH and IL is not easily determined, the sense amplifier of FIG. 1 may be not sensitive enough and easily affected by noise.
FIG. 2 is a circuit diagram of a conventional sense amplifier of U.S. Pat. No. 6,762,953. In FIG. 2, the outputs 22 and 23 of comparator 21 respectively receives current (Iref−Icell) and current (Icell−Iref), thus the current sensing range is larger than the current sensing range of the sense amplifier of FIG. 1 (In FIG. 1, the current sensing range is Iref−Icell). The reference current Iref of FIG. 2 is
            1      2        ⁢          (                        I          H                +                  I          L                    )        ,however, thus a half divider circuit is required or the current mirror of the sense amplifier of FIG. 2 is utilized to output half of the current IH and IL. Although the current sensing range of FIG. 2 is twice the current sensing range of the sense amplifier of FIG. 1, the sense amplifier of FIG. 2 requires more transistors and greater layout area. Thus, a sense amplifier with a larger voltage sensing range and simple circuit design is desirable.